Tannin extraction

Abstract

PCT No. PCT/AU90/00335 Sec. 371 Date Apr. 2, 1991 Sec. 102(e) Date Apr. 2, 1991 PCT Filed Aug. 7, 1990 PCT Pub. No. WO91/01989 PCT Pub. Date Feb. 21, 1991.Method for extracting tannins from bark and multi-stage apparatus suitable therefor. The method includes the steps of: contracting bark containing tannins with a first tannin enriched aqueous wash medium at elevated temperatures; separating the tannin-rich liquid extract by filtration including dewatering the resulting filter cake under pressure; washing the filter cake with a plurality of aqueous wash media at elevated temperatures; and separating the tannin enriched media after each washing stage by filtration including dewatering the filter cake under pressure; wherein each successive wash medium has a lower degree of tannin enrichment than the preceding wash medium.

I claim: 
     
       1. A method for extracting tannins from tannin-containing bark; said method comprising the steps of: (a) contacting tannin-containing bark with an amount of a first tannin-enriched aqueous wash medium for a length of time sufficient to extract the tannins to form a slurry of a tannin-rich liquid extract and extracted bark; said tannin-enriched aqueous wash medium having a temperature within the range of 60°-100° C.;   (b) separating the tannin-rich liquid extract from the extracted bark by pressure filtration; said step of separating comprising: (i) filtering the tannin-rich liquid extract from the extracted bark to form a filter cake of extracted bark and retained tannin-rich liquid; and   (ii) dewatering the resulting filter cake under a sufficient pressure to decrease the amount of retained tannin-rich liquid in the filter cake;     (c) subjecting the filter cake to two subsequent washing and separating stages, wherein: (i) each washing stage comprises washing the filter cake with an amount of a tannin-enriched aqueous wash medium having a temperature of 60°-100° C. to extract therein tannin remaining in the filter cake and to form a tannin-rich liquid extract; said tannin-enriched aqueous wash medium having a lower degree of tannin enrichment than each previous tannin-enriched aqueous wash medium; and   (ii) each separating stage comprises separating the tannin-rich liquid extract from the filter cake after each washing step by pressure filtration; said step of separating comprising:   (A) filtering the tannin-rich liquid extract from the filter cake; and   (B) dewatering the filter cake under a sufficient pressure to decrease the amount of retained tannin-rich liquid in the filter cake; and     (d) collecting the tannin-rich liquid extract resulting from each washing and separating stages.   
     
     
       2. The method according to claim 1, further comprising a step of washing the filter cake with water after the step of subjecting the filter cake to two subsequent washing and separating stages to extract therein tannin remaining in the filter cake; said water having a temperature within the range of 60° C. to about 100° C. and containing no tannins. 
     
     
       3. The method according to claim 1, wherein the first tannin-enriched aqueous wash medium contains a portion of the tannin-rich liquid extract. 
     
     
       4. The method according to claim 1, wherein the tannin-rich liquid extract is subjected to concentration by evaporation to form a concentrated extract, 
     
     
       5. The method according to claim 1, where the tannin-containing bark comprises Pinus Radiata bark. 
     
     
       6. The method according to claim 1, wherein the tannin-containing bark is comminuted before contact with the first tannin-enriched aqueous wash medium. 
     
     
       7. The method according to claim 6, wherein no more than 5% of the tannin-containing bark is greater than about 1 mm in any dimension. 
     
     
       8. The method according to claim 1, wherein each step of dewatering the filter cake is conducted under a pressure of at least about 10 bar to about 16 bar. 
     
     
       9. The method according to claim 1 wherein the tannin-containing bark and the first tannin-enriched aqueous wash medium are slurried at a temperature within the range of about 60° to 100° C. for a period of 10 minutes to 1 hour. 
     
     
       10. The method according to claim 1, wherein the filter cake is subjected to two to four subsequent washing and separating stages. 
     
     
       11. The method according to claim 10, wherein each washing and separating stage occurs within a filter without displacement of the filter cake from the filter. 
     
     
       12. A method for extracting tannins from tannin-containing bark; said method comprising the steps of: (a) contacting tannin-containing bark with an amount of a first tannin-enriched aqueous wash medium at a temperature of 60°-100° C. for a length of time sufficient to extract the tannins to form a slurry of a first tannin-rich liquid extract and extracted bark;   (b) separating the first tannin-rich liquid extract from the extracted bark; said step of separating comprising: (i) filtering the first tannin-rich liquid extract from the extracted bark with a pressure filter to form a filter cake of extracted bark and retained tannin-rich liquid; and   (ii) dewatering the resulting filter cake under a pressure within the range of about 10 bar to a maximum pressure that the presure filter can withstand to decrease the amount of retained tannin-rich liquid in the filter cake;     (c) washing the filter cake with an amount of a second tannin-enriched aqueous wash medium at a temperature of 60°-100° C. to extract the tannins to form a second tannin-rich liquid extract; said second tannin-enriched aqueous wash medium having a lower degree of tannin enrichment than said first tannin-enriched aqueous wash medium;   (d) separating the second tannin-rich liquid extract from the filter cake; said step of separating comprising: (i) filtering the second tannin-rich liquid extract from the filter cake with the pressure filter; and   (ii) dewatering the filter cake under a pressure within the range of about 10 bar to a maximum pressure that the pressure filter can withstand to decrease the amount of retained tannin-rich liquid in the filter cake;     (e) washing the filter cake with an amount of a third tannin-enriched aqueous wash medium at a temperature of 60° to 100° C. to extract the tannins to form a third tannin-rich liquid extract; said third tannin-enriched aqueous wash medium having a lower degree of tannin enrichment than said second tannin-enriched aqueous wash medium;   (f) separating the third tannin-rich liquid extract from the filter cake; said step of separating comprising: (i) filtering the third tannin-rich liquid extract from the filter cake with the pressure filter; and   (ii) dewatering the filter cake under a pressure within the range of about 10 bar to a maximum pressure that the pressure filter can withstand to decrease the amount of retained tannin-rich liquid in the filter cake;     (g) washing the filter cake with an amount of water containing no tannin therein at a temperature within the range of 60° C. to 100° C. to extract the tannins to form a final liquid extract; and   (h) separating the final liquid extract from the filter cake; said step of separating comprising: (i) filtering the final liquid extract from the filter cake with a pressure filter; and   (ii) dewatering the filter cake under a pressure within the range of about 10 bar to a maximum pressure that the pressure filter can withstand to decrease the amount of retained liquid in the filter cake; and     (i) collecting the first tannin-rich liquid extract, second tannin-rich liquid extract, third tannin-rich extract, and final tannin-rich extract.   
     
     
       13. The method according to claim 12, wherein the pressure is within the range of about 10 bar to about 16 bar. 
     
     
       14. An apparatus for extracting tannins from tannin-containing bark comprising: (a) a first extraction tank for slurrying tannin-containing bark with a tannin-enriched aqueous wash medium to form a slurry of a tannin-rich liquid extract and extracted bark, said tank having a means for adding the tannin-containing bark, a means for adding the tannin-enriched aqueous wash medium, and a means for discharging the slurry;   (b) a pressure filter connected to the means for discharging the slurry for fitering the discharge slurry to separate the tannin-rich liquid extract from the extracted bark to form a filter cake and to dewater the filter cake, said pressure filter having a means for applying pressure on the filter cake;   (c) one wash liquor holding tank connected to the pressure filter for providing a tannin-enriched aqueous wash medium for washing the filter cake on the filter to form a tannin-rich liquid extract;   (d) a source of water connected to the pressure filter for providing a final medium for washing the filter cake in the filter to form a tannin-rich liquid extract;   (e) a tannin-rich extract collection tank for holding the tannin-rich liquid extracts flowing from the pressure filter and connected to the pressure filter; and   (f) pumping means coupled to the tanks and the pressure filter for moving the slurry and the liquid media between the tanks and the pressure filter.   
     
     
       15. An apparatus according to claim 14, wherein the extraction tank includes agitating means for retaining the slurry in suspension. 
     
     
       16. An apparatus according to claim 14 further comprising a tannin-rich recycle circuit connected to the pressure filter and the extraction tank and adapted for separating a tannin-rich stream flowing from the pressure filter and for recycling a portion of the stream to the extraction tank. 
     
     
       17. An apparatus according to claim 14 in which the means for applying pressure on the filter cake applies a pressure within the range of about 10 bar to a maximum pressure that the pressure filter can withstand. 
     
     
       18. An apparatus according to claim 17, wherein the means for applying pressure on the filter cake applies a pressure within a range of about 10 bar to about 16 bar. 
     
     
       19. An apparatus according to claim 14, further comprising a second extraction tank for slurrying and aging tannin-containing bark and a tannin-enriched aqueous wash medium while the contents of the first extraction tank undergo separation and washing, said second extraction tank having means for adding tanning-containing bark, a means adding a tannin-enriched aqueous wash medium, and a means for discharging the slurry connected to the pressure filter. 
     
     
       20. The apparatus according to claim 14, further comprising three wash liquor holding tanks connected to the pressure filter for providing wash media of successively lower tannin contents. 
     
     
       21. An apparatus according to claim 14 further comprising an evaporator connected to the tannin-rich extract collection tank and adapted to form a concentrated extract from the tannin-rich liquid extract.